In prior locomotive drive systems including an engine driven three-phase alternator and two or more DC traction motors, it has been known to wind the alternator with two or more separate sets of output (stator) windings and to separately rectify the generated outputs for application to respective traction motor windings. In such systems, circumferentially adjacent stator coils are generally connected in series to define phase winding components, and such phase winding components are generally distributed symmetrically around the stator so that one of the sets of three-phase windings can be loaded while the other is unloaded, if desired. A system of this type is shown and described in the U.S. Pat. No. 3,340,448 to Thiessen which patent was issued on Sept. 5, 1967, and is assigned to the assignee of the present invention.
Twin winding three-phase alternators also find application to drive systems incorporating AC traction motors. In such a system, each winding set is connected to supply AC power to one or more AC traction motors through an AC/DC converter and a DC/AC inverter. An electronic control unit responsive to various motor and demand parameters controls the gating of thyristors within the inverters and converters to individually regulate the tractive effort of the motors. In this type of drive system, reliable control of the converter and inverter thyristors requires that magnetic coupling between the two sets of alternator windings be minimized. In other words, the control unit is sensitive to transient voltages induced in one winding set by current interruption or changes in the other winding set, and may make an erroneous control response thereto.
Magnetic coupling between the winding sets of a twin winding alternator is primarily due to slot leakage between winding elements disposed in the same stator slot. Accordingly, the magnetic coupling between winding sets can be minimized if any given stator slot contains winding elements of only one or the other of the winding sets. However, this objective cannot be achieved with conventional three-phase winding arrangements wherein circumferentially adjacent coils are connected in series to define phase winding components unless the winding pitch is made relatively low. Low winding pitch reduces the alternator output voltage and is therefore undesirable.